2007 update of probabilistic world population projections

An article based on the new 2007 IIASA Probabilistic World Population Projections has recently been published in “Nature” under the title “The coming acceleration of global population ageing”.

The detailed results for ten indicators for individual years from 2008 to 2100 for 13 world regions as well as for the world as a whole is given in the following ten Excel files:

Total Population
Mean Age
Proportion Below Age 20
Proportion 20-60
Proportion Above Age 60
Proportion Above Age 80
Old Age Dependency Ratio (60+ / 20-60)
Life Expectancy Females
Life Expectancy Males
Total Fertility Rate (TFR)

We recommend the following citation for users of these data:

Wolfgang Lutz, Warren Sanderson and Sergei Scherbov, IIASA’s 2007 Probabilistic World Population Projections, IIASA World Population Program Online Data Base of Results 2008, http://www.iiasa.ac.at/Research/POP/proj07/index.html?sb=5

The following summary of projection results was recently published in IIASA's magazine Options:

In the view of the outside world, population projections are the single most important result of the work of demographers. Our projections are a key component of policy planning, in areas as diverse as pension reforms, school reforms, regional planning, or the macroeconomic models of ECOFIN (EU Council of Economics and Finance Ministers) and the global assessments of IPCC (Intergovernmental Panel on Climate Change). These users of demographic projections not only need a best-guess future population path. They increasingly request explicit information about the uncertainty involved in future demographic trends. This is because deviations may have massive financial implications such as an underestimation of the speed of aging for the financing of pay-as-you-go pension systems. Only truly probabilistic population projections can serve this purpose. The conventional high–medium–low variants (based on alternative fertility assumptions) can greatly underestimate the uncertainty in the future number of elderly because they disregard old-age mortality uncertainty. A larger set of scenarios also does not fully serve this purpose, if it makes no reference to a probability range. But what should these assumptions be based on? Mechanistic trend or error extrapolations without substantive reasoning are not enough. Science-based projections require a better knowledge base. We need more hypotheses and theories with predictive power that also can be tested. This requires nothing less than a reorientation of the currently dominant demographic research agenda.

Article from IIASA's Options (Winter 2007)
Figure 1. Uncertainty distribution of total world population to 2100, in billions.

The growing divergence in population trends and concerns

Not surprisingly, the global demographic landscape is difficult for many people to comprehend. In some parts of the world, rapid population growth continues to be a major source of concern, with populations likely to triple in some places over the course of this century. In some areas—predominantly Eastern Europe—the population has actually started to decline, causing great concern among governments and the public there.

2001 Projections “The End of World Population Growth,” the set of probabilistic world population projections published by IIASA’s World Population Program (POP) in Nature in 2001, was generally greeted as good news for the planet. According to POP’s research, the clearly unsustainable growth in human numbers in a finite environment would reach a benign end through voluntary family limitation rather than the often-predicted Malthusian check of increasing death rates. While the findings were cause for optimism at the global level, they were partly misinterpreted. In fact, some people believed that population concerns at the regional and national level were now no longer justified. Nothing could be further from the truth. Indeed, for different reasons in different parts of the world, concern over population is rising. In Africa, there is mounting evidence that continuing “explosive” population growth is a key obstacle to the eradication of poverty and to improving education. At the same time, countries with rapidly aging and shrinking populations worry that their social security systems will be overburdened and that the welfare of their citizens is at risk. This bifurcation in terms of demographic trends and associated concerns has actually become more pronounced since 2001.

New Projections In mid-2007 POP produced a new set of world population projections using the same long-term assumptions about the future levels of fertility, mortality, and migration as in the 2001 study, but incorporating all the new empirical evidence that has become available between 2001 and now. Trajectories of population growth have a strong path dependence. Thus, an extra six years of empirical trends at the beginning of the century have the potential to greatly influence the range of likely outcomes by the end of it. The new results show that there are indeed significant changes in the outlook for certain regions, but as these go in opposing directions and partly offset each other, the aggregate results at the global level remain amazingly stable. The probability that world population will peak during this century has increased marginally. The period during which the median of the projections reaches a peak (around 2070) and the level of this peak (around 9 billion people) remain virtually unchanged. As Figure 1 indicates, by the middle of the century, the 80 percent uncertainty range for world population is 7.8 to 9.9 billion. By 2100 it further broadens to 6.2–11.1 billion. In other words, there is a more than a 10 percent chance that the world population in 2100 will be smaller than it is today and an equal chance it could be more than 11 billion. However, a further doubling of world population from currently 6.6 to 13.2 billion is seen as extremely unlikely (a less than 2 percent chance) from today’s perspective.

Regional Shifts This global picture, however, hides several important regional shifts. Because of continued very low fertility or further fertility declines in some regions that already had low fertility, Eastern Europe and the China region are now shown by the POP research as having lower population growth than was projected in 2001. This is offset at the global level by higher anticipated population growth in sub-Saharan Africa. In the latter, two interesting recent developments have critically changed the earlier assumptions made by all international forecasting agencies. First of all, the fertility decline seems to have stalled in a number of important African countries. This coincides with an actual decline in the level of schooling of young adults and worsening health care and family planning services. Second, fewer people than anticipated are dying of AIDS. This has less to do with the rapid spread of anti-retroviral treatment in parts of the continent than with a significant downward correction of the estimates made by UNAIDS of the numbers of people infected with the virus. Put together, a higher starting level of fertility plus a lower level of mortality result in higher population growth, even when the long-term assumptions are left unchanged.

Africa’s Population to Double As Figure 2 shows, Africa’s population will almost certainly more than double from its current level of around 740 million. Because of the great longerterm uncertainties surrounding the future speed of fertility decline and the possible new health crises under the very poor development conditions anticipated, the 95 percent range by the end of the century is very broad, from a low 1.1 billion to a very high 3.3 billion. The central 20 percent range is 1.9–2.2 billion by 2100. Two factors will in all likelihood keep Africa at the bottom of world development unless some trends change radically in the near future: continued very rapid population growth together with stagnant or declining educational attainment levels (partly as a consequence of rapidly increasing numbers of children), and the additional environmental and agricultural problems likely to be caused by climate change.

Figure2. Sub-Saharan Africa population, in millions.

Eastern Europe's Population Shrinking Eastern Europe and the European part of the former Soviet Union lie at the other extreme. The political changes that took place in 1990 triggered a precipitous fertility decline, together with some mortality increase, particularly among men. However, these regions already had a fairly old age structure. In 2001 it was assumed that these extremely low levels of fertility (in some cases close to half the replacement level) were only a temporary distortion. Since then, there has been continued low fertility (together with significant out-migration in most of the countries). This has distorted the age structure to such an extent that even in the unlikely event of a return to replacementlevel fertility, there would be a population decline because fewer and fewer women are entering reproductive age. In 2001 we were already projecting a significant shrinkage for these regions. However, as is evident from Figure 3, the shrinking is faster and more dramatic. A reduction over the coming decades is a near certainty, and the population size is likely to decline to less than half its current level by the end of the century.

Figure 3. Eastern European population, in millions.

China: First Growth Then Shrinkage Finally, China as a region is a very interesting case. It combines near certain population growth in the next decades with almost certain population decline in the longer run. As shown in Figure 4, the history of quite high fertility followed by a very steep fertility decline has resulted in an age structure where age groups of people of reproductive age are still increasing. Currently, this implies some further population growth to around 1.5 billion in the 2020s before a lasting population decline will begin. The population outlook for China is made more uncertain because there is no consensus among experts as to what the current level of fertility is. In 2001 the general agreement was that the total fertility rate was around 1.9. Since then, there has been mounting evidence that even at the time of the 2000 census it was already much lower—somewhere in the 1.2 to 1.8 range, with the best guess at around 1.5. To cope with this uncertainty regarding the world’s most populous country, POP chose to expand the probabilistic approach to include not only uncertain future paths but also uncertain starting conditions above the range just indicated. But as the median of this range is still 0.4 children lower than the earlier assumptions, the new outlook for China shows more rapid population aging and shrinkage than just a few years ago, again under the same long-term assumptions. As Figure 4 shows, after an initial increase China’s population is likely to be back down to its 2000 level during the 2040s and then, by the end of the century, possibly almost down to half the 2000 level. Population projections have traditionally assumed that all countries of the world, after passing through the demographic transition, will converge demographically. This has been most obvious in the long-held United Nations assumptions that all countries would converge to replacement-level fertility and eventually even to the same level of life expectancy; moreover, as a result, demographic differentials around the world would disappear. But on a global level the demographic trends have seen little convergence over the past decades. In fact, over the past few years there has been outright divergence. Regions with already low fertility have seen further declines, and regions with high fertility have shown lower than expected declines. The high level of path dependency inherent in population growth has already produced a more heterogeneous demographic picture for the twenty-first century. Based on these trends one may also have to rethink, among other things, some of the longer term socio-economic scenarios used for the analysis of climate change. A world where there is little international cooperation may not necessarily be a world with a higher population; it may be a world with rapid population shrinkages in some areas and explosive growth in others.

Figure 4. Population of China and Cambodia, Hong Kong, Laos, Mongolia, North Korea, Taiwan, Vietnam, in billions.

Further information IIASA’s World Population Program at www.iiasa.ac.at/Research/POP
Professor Wolfgang Lutz is the leader of IIASA’s World Population Program. Professor Warren Sanderson and Dr. Sergei Scherbov are senior research scholars in IIASA’s World Population Program.

Responsible for this page: Suchitra Subramanian
Last updated: 30 Jan 2012